1. Field of the Invention
The present invention relates to a method for prevention of increase in particles in copolymer for semiconductor resist, which is applied to a semiconductor resist suitably used for formation of a resist film used in semiconductor lithography.
2. Description of the Prior Art
In the lithography employed for production of semiconductor, formation of finer pattern is required with the increase in the integrity of semiconductor. A light source of shorter wavelength is essential for the finer pattern. Currently, a lithography using a crypton fluoride (KrF) excimer laser beam (wavelength: 248 nm) is becoming a main stream and a lithography giving a line width of 100 nm or less using an argon fluoride (ArF) excimer laser beam (wavelength: 193 nm) is being put to practical use. Further, short-wavelength radiation lithography techniques using a fluorine dimer (F2) excimer laser beam (wavelength: 157 nm), an extreme ultraviolet light (EUV), an X-ray, an electron beam or the like are being developed.
In these semiconductor lithographies, there are used a resist film for forming a resist pattern (to be transferred onto a substrate) utilizing the film's property of change in solubility in alkali developing solution by the action of acid, and various coating films formed on or beneath the resist film. As the coating film formed beneath the resist film, i.e. the underlayer film, there can be mentioned, for example, an anti-reflection film used for suppressing the light reflected from a substrate and allowing for the precise formation of a fine resist pattern; a flattening film used as an underlayer film of resist in order to smoothen the surface unevenness of a pattern-formed substrate when another resist pattern is going to be formed on the substrate; and an underlayer film in a multi-layered resist, used for transferring a resist pattern by dry-etching.
These coating films are formed by dissolving a polymer for lithography (having the properties of an intended coating film) and additives, in an organic solvent to prepare a coating solution, coating the solution on a substrate by spin coating or the like, and as necessary applying a heat or the like to remove the solvent. The copolymer for lithography used needs to have optical and chemical properties required for resist film or anti-reflection film; physical properties such as coatability, adhesivity to substrate or underayer film, and the like; and basic properties to be possessed by a copolymer for coating film, such as no presence of dust which hinders formation of fine pattern.
As the copolymer used in the resist film, there are a negative type whose solubility in alkali developing solution decreases when an acid acts thereon, and a positive type whose solubility in alkali developing solution increases when an acid acts thereon. A positive type resist copolymer contains a recurring unit having a polar group by which the copolymer can have high adhesivity to semiconductor substrate or to underlayer film and can control solubility in organic solvent for resist or in alkali developing solution; a recurring unit having such a structure that a non-polar substituent is dissociated by the action of an acid to generate a polar group soluble in alkali developing solution (these two recurring units are essential); and, as necessary, a recurring unit having an acid-stable non-polar substituent by which the copolymer can control solubility in organic solvent for resist or in alkali developing solution.
As specific examples of the positive type resist copolymer, there are known, in KrF lithography, copolymers containing a recurring unit derived from hydroxystyrene and a recurring unit derived from an acid-decomposable alkoxystyrene; copolymers containing a recurring unit derived from hydroxystyrene and a recurring unit derived from an acid-decomposable alkyl (meth)acrylate; and polymers wherein part of the hydroxystyrene-derived recurring unit has been protected with an acetal. In ArF lithography, there are known, for example, copolymers containing a recurring unit derived from a (meth)acrylate substituted by a hydroxyalkyl group and a recurring unit derived from an acid-decomposable alkyl (meth)acrylate. As the resist pattern has become finer, there are coming to be used those resins wherein a highly hydrophobic alicyclic structure is introduced as an acid-dissociable group and a contrast of dissolution in alkali developing solution before and after dissociation is increased, or those resins wherein etching resistance or light transmittance is increased.
Meanwhile, in the semiconductor lithography, defects of resist pattern have an influence on the yield of the semiconductor produced; therefore, as the resist pattern has become finer, it is required to decrease even the fine defects. In particular, high-molecular compounds of extremely high molecular weight contained in a copolymer for resist in a very small amount (hereinafter, such compounds are referred to as high polymers) and compounds which are a compound containing recurring units having a polar group or an alicyclic structure and wherein the same recurring units are connected continuously to form a long chain (hereinafter, such compounds are referred to as long consecutive sequence), are hardly soluble in an organic solvent for resist or in an alkali developing solution. Such high polymers or long consecutive sequence, even if they are dissolved right after production of a resist solution containing a copolymer, tend to separate out, during the storage of the resist solution, as a foreign matter of very small particle diameter (hereinafter, the foreign matter is referred to as particles) (it is presumed that, in separating-out of the particles, an extremely small, hardly-soluble substance acts as a nucleus); therefore, there is a problem in that defects of resist pattern appear easily after development (hereinafter, these defects are referred to as defects after development) and the problem has been a big obstacle in obtaining a finer resist pattern.
As the method for removing the very small dust contained in a copolymer having an alicyclic structure, there is known, for example, a method (Patent Literature 1: JP-A-1999-231539) which comprises filtering, through a filter having a pore diameter of 0.1 μm or less, a resist composition obtained by dissolving a copolymer and a radiation-sensitive compound (e.g. a photo acid generator) in a solvent. With this method, the very small dust present at the time of filtering is removable; however, there has remained unachieved a task of reducing the particles which separate out with the passage of time during the storage of filtered resist composition.
Meanwhile, there is known a method (Patent Literature 2: JP-A-2001-350266) which comprises passing, through a filter having a positive zeta-potential, a resist composition obtained by dissolving a binder component (e.g. an alkali-soluble resin or a resin which becomes alkali-soluble by the action of an acid) and an acid-generating agent in a solvent, to produce a storage stability-superior resist composition which, even when stored for a long period and coated on a silicon wafer, gives a thin film free from any pin hole defect. However, when such a composition containing an acid-generating agent is passed through a filter having a positive zeta-potential, part of the ionic additive such as acid-generating agent or the like is captured by the filter having a positive zeta-potential; consequently, there has been a problem of a change in lithography properties (e.g. sensitivity) and the method is not practical. Further, probably because there is no sufficient capture of nucleic substance of particles which causes defects after development, there has been a problem in that it is impossible to reduce the particles which separate out with the passage of time during the storage of filtered resist composition.
As the method of passing a polymer solution for lithography containing no ionic additive, through a filter having a zeta-potential, there are known a method (Patent Literature 3: JP-A-1996-165313) which comprises passing a vinylphenol-based polymer dissolved in a solvent, through a filter which produces a zeta-potential in the presence of a cation-controlling agent, and a method (Patent Literature 4: JP-A-1998-237125) which comprise passing a vinylphenol-based polymer dissolved in a solvent through a filter containing an ion exchanger and/or a chelate-forming compound and a cation-controlling agent in combination or separately, to effectively reduce the metal impurities contained in a vinylphenol-based polymer dissolved in a solvent. With these methods, however, there has been no sufficient reduction in particles which cause a problem particularly in a copolymer having an alicyclic structure.
Some resist compositions show a change in sensitivity with the passage of time. As the method for removing a substance which causes such a sensitivity change, there are known a method (Patent Literature 5: JP-A-2001-125269) which comprises filtering a resist solution containing an acid-degradable resin and an acid-generating agent through a filter containing a polyethylene, a nylon or a polysulfone, and a method (Patent Literature 6: JP-A-2003-330202) which comprises filtering a resin of alicyclic (monocyclic or polycyclic) hydrocarbon structure whose solubility in alkali developing solution increases when an acid acts thereon, through an ion exchange filter and/or an undissolved colloid-removing filter. With these methods, the sensitivity change is suppressed; however, the effect for removal of nucleic substance (hardly-soluble dust) is unclear and the problem of dust generation with the passage of time during storage remains undissolved.